1. Field of the invention
The present invention generally relates to a fixing apparatus for use in an electrophotographic equipment, such as copier, printer, facsimile and the like, to fuse and press an unfixed toner or an unfixed toner image formed on a recording sheet, such as paper and the like, and to fix the toner to the sheet permanently.
2. Prior Art
For a fixing apparatus of electrophotographic equipment, a roller system, so called two rollers system, has been applied, which comprises a fixing roller containing a heating source and a pressing roller pressed against the fixing roller with a particular pressure. In such a roller system, various related patent applications have been filed and this system has been widely used. In recent years, a new fixing system, so called a belt fixing system, has been proposed and has also been offered for practical use as more advanced fixing system than the two rollers system. Specifically, this system comprises a fixing roller containing no heating source, a heating roller containing a heating source, a fixing belt as sort of endless type which is extended from the fixing roller to the heating roller with tension, and a pressing roller adapted to press the fixing roller through the fixing belt.
Whenever any conventional systems having such constructions are used, it is essential to supply a suitable amount of heat to an unfixed toner or toner image supported on a sheet for fusing it, that is, the unfixed toner should be adequately heated and fused. In addition, a particular pressure must also be applied to the unfixed toner, which has been fused, for fixing it on the sheet, that is, the unfixed toner fused should adequately be pressed.
Thus, in any conventional systems, the fixing roller and the pressing are pressed each other with a particular pressure to form a nip portion therebetween and a sheet supporting an unfixed toner is heated and pressed by the nip when the sheet is passed through the nip portion.
However, in any fixing apparatuses of the conventional systems, when the applied pressure at the nip portion (hereinafter, refer to nip pressure) was insufficient, there causes a problem such that a toner image would not be fixed on a sheet, e.g. the toner image would be fallen off from the sheet by touching the image with hand, due to the offset effect under low pressure. Thus the nip pressure is arranged with relatively high value for ensuring to eliminate the offset effect under low pressure. Specifically, the nip pressure is approximately 200 kPa even in a low speed type fixing apparatus, and the pressure should be increased up to approximately 250 k Pa in an intermediate or high-speed type fixing apparatus.
Since the nip pressure is arranged in the relatively high pressure, the pressure acts to a driving unit for transferring a sheet as driving resistance so that the driving unit must be enforced to drive with higher torque. Consequently, a high torque type driving motor, which is expense, must be equipped as a driving source for the driving unit and the transmission mechanism of the driving unit also requires higher quality on durability for the higher transmitted torque in the driving unit. This results in cost overrun.
Further, since the driving unit is driven with high torque, it may be hard to maintain effective lifetime of each components of the driving unit. Thus, it is desired to improve the effective lifetime.
Furthermore, in such conventional fixing apparatus using rollers, since the major diameter and/or the radial thickness of the roller requires to be larger in order to prevent bending of the roller, the size and weight of the apparatus are increased. It is also desired to improve this problem.